The present invention relates to a method for preventing, removing, or controlling fouling deposits.
Fouling deposits build up on equipment surfaces, reducing performance of the equipment. An example of this is corrosion of pipelines, storage tanks, and the like, in which chemicals such as hydrocarbons are stored or transported. Another example is corrosive attack and change of appearance of metals and alloys as a result of protracted exposure to atmospheric weather and other environments. Yet another example is accumulation of biological slime on heat transfer surfaces or in pipelines.
In the case of pipelines used for transporting chemicals, internal corrosion often is worst on the bottom of lines where water and soil particles may accumulate. With respect to transport of petroleum liquids by pipepline, it is virtually impossible to keep water out of the liquid, as water can enter the system wherever there is exposure to the atmosphere, such as in storage tanks. In the case of crude oils, it is not practical or economic to remove all of the water from the oil before transmitting it to the refinery.
To prevent such corrosion, it is desirable that a relatively high concentration of a suitable corrosion inhibitor be present at the points where water and scale particles tend to accumulate. By and large, it is not practical to accomplish this result merely by adding an inhibitor in conventional liquid or powdered from to the oil in the pipeline. If conventional oil soluble inhibitors are used, they would remain in the oil phase and not pass into the water phase. If conventional water soluble inhibitors were added to the oil, they would remain dispersed in the oil phase and would enter the separated water phase only to a small extent if at all.
Exterior coatings on metals and alloys to protect from corrosion and fouling are also impractical, as the coatings are not permanent and often mask the desired properties of the substrate coated.
In the case of biological slime building up on equipment surfaces, chemicals such as chlorine or non-oxidizing biocides are fed into process water to remove fouling deposits. Because of mass transport limitations in the liquid phase or reaction limitations, much of the chemical is consumed by reactions in the fluid phase before it can react with the deposit. In some case, particularly with chlorine addition, some of the chemical is flashed off and is thus wasted. These processes waste chemicals or release much of the chemical to the environment, both of which are undesirable. Problems of this type discourage use of certain chemicals in such applications even though they may be very effective in removing or preventing the fouling deposit.
A number of different methods have been devised to deal with this problem, although none of them has been entirely successful.
Williams, in U.S. Pat. No. 2,795,560, discloses a method for preventing corrosion in the pipelines transporting petroleum products by introducing a plurality of solid pellets containing a water soluble, oil insoluble corrosion inhibitor within the pipeline. These pellets travel along with the fluid, and have a density greater than the hydrocarbon fluid and at least as great as water.
Fuchs, in U.S. Pat. No. 3,338,670, discloses a class of polyphosphate glasses which have both metal corrosion-inhibiting properties and calcium sequestering properties, and which have a controlled rate of dissolution in aqueous solutions. These glasses can be added to aqueous solutions to minimize corrosion and scaling.
Breton, in U.S. Pat. No. 3,205,566, discloses corrosion inhibited metal-glass compositions which are primarily metallic in appearance and properties, but which are inhibited against corrosion by intimate association with a moisture-leachable glass which, during its slow dissolution, releases corrosion-inhibiting substances.
Koury, in U.S. Pat. No. 3,470,001, discloses a gel-like composition which encapsulates essential ingredients such as corrosion inhibitors. This gel can be used as a protective coating for a metallic surface.
Miale et al., in U.S. Pat. No. 4,253,877 disclose a method for controlling fouling in marine substrates by painting the substrate with a paint containing microencapsulated anti-fouling agents.